Low-pressure hydrocephalus is an extremely rare condition. Authors of the few previous reports on this subject have suggested that symptoms are gradual in onset and successfully treated. In this report the authors presented two unique cases of very low pressure hydrocephalus in which the patients experienced rapid deterioration requiring negative-pressure cerebrospinal fluid drainage to achieve the best possible neurological function; outcomes in both patients ultimately were poor. The constellation of findings suggests that this may be a distinct clinical entity.